On the surfaces of home electric appliances, stationery products or automotive interior products, grains are formed for various purposes, such as improvements in appearance and tactile sensation, glare proofing, and anti-skidding. Various fine patterns, such as leather-grain, wood-grain, rock-grain, sand-grain, pear-grain, and geometric patterns, are adopted as grain patterns. For the purpose of forming these grains on the surfaces of resin products, grain patterns are conventionally given on metal molds, for example, by an etching method or an electroforming method.
In the above-mentioned etching method, shapes are formed by corrosion; hence, the method is also applicable to metal molds for producing large products at low cost; however, the method is difficult to represent fine shapes and also difficult to form the same shape repeatedly.
Furthermore, in the case that a grain is formed in the normal direction of the curved surface of a product, if so-called undercut, in which the removing direction of the product intersects with the curved surface of the product on a surface along the opening direction of an injection molding metal mode, occurs, the unevenness of the grain on the mold interferes with the unevenness of the grain transferred to a molded product when the molded product is removed; if the molded product is forcibly removed from the metal mold, the grain on the molded product is damaged. To avoid this, it is necessary to prevent the occurrence of the undercut by decreasing the depth of the grain depending on the change in draft as the draft (the supplementary angle of 90° formed by the normal direction of the curved surface of the product and the removing direction of the product) along the opening direction of the mold becomes a range closer to 0°.
Moreover, in the case that a grain pattern is given to a metal mold by the etching method, the depth of the grain cannot be changed continuously; if the depth is made shallow stepwise, the joints thereof are exposed and the appearance is damaged.
On the other hand, in the electroforming method, a thin resin sheet on which a grain is embossed is bonded by hand work to the surface of a model that is formed into the shape of a product, and this is used as a master model and is subjected to resin inversion and electroforming processes to obtain a forming mold. However, when the resin sheet is bonded to a three-dimensional model, elongation or distortion occurs, or nonconformity occurs in the patterns at the joints; hence, the worker requires a lot of skill to make modifications so that these are less noticeable. Besides, due to the result that the method requires many processes, the work period is long and the cost is high; hence, only the special manufacturers having dedicated facilities can adopt the electroforming method. What's more, the shape and size of the grain are determined by the grain on the resin sheet that is bonded when the master model is formed, and it is thus difficult to provide additional patterns at any desired portions later and also difficult to expand or reduce the grain patterns.
Still further, both the methods are disadvantageous in that chemical solution processing or the like is required.
In addition, for the purpose of making it possible to form high-quality grains more easily, a known method has been proposed in Patent Document 1 and Patent Document 2, for example. According to this method, the surface measurement values obtained by reading the surface shape of a leather model or the like are converted into image data in which the depth of the surface shape is represented by the density of 256 gradations, whereby the shape data of the grain pattern is digitized; on the basis of this, processing data is generated using a computer, and by using this processing data, a grain is given to a planar structure, such as an emboss roll or an emboss plate, or to a stereoscopic structure, such as a metal mold, by cutting or laser processing.
When the processing data is generated from the grain shape data, if the processing data is obtained simply by performing projection onto the surface of a stereoscopic structure having a three-dimensional shape, such as a metal mold, individual grain portions are expanded and distorted on the inclined surfaces of the stereoscopic structure; hence, the grain is required to be formed in the normal direction on the curved surface of the product.
For this reason, for the purpose of forming a grain in the normal direction of the surface of a product, modeling software is provided in which voxel data conversion for generating a grain by overlaying voxels on the curved surface of the product is used depending on the density corresponding to the depth of the grain in image data, for example.
In this voxel system, product shape data is converted into voxel data composed of a small sphere or cube, and the pixels of the image data of the grain corresponding to the respective voxel data are determined. Then, the densities of the pixels are converted into displacement amounts, and voxels corresponding to the displacement amounts are overlaid on the voxel data. These are converted into polygon data and final data is obtained.